1. Field of the Invention
This invention relates to double heterostructure (DH) light emitting diodes, and more particularly, to DH junction lasers and spontaneous emission diodes wherein the p-n junction is separated from the active region.
2. Description of the Prior Art
The advancement of optical communications technology depends on the development of inexpensive and reliable components. A major breakthrough in this technology was the development of semiconductor DH junction lasers for use as signal generators. The DH laser structure, which basically comprises a narrow bandgap active region sandwiched between relatively wider bandgap, lattice-matched cladding layers, brought about a dramatic decrease in the laser oscillation initiation current (threshold) which made CW operation at room temperature possible. The success of the DH structure is due to confinement of light and carriers perpendicular to the junction plane by the heterojunctions formed at the interfaces between the narrow and the wide bandgap layers. Various device structures have been proposed which enable lateral control of carriers and light in a direction parallel to the junction plane. These structures further reduce threshold current and are often conducive to oscillation in the lower order modes.
Referring to FIG. 1, an illustrative prior art Al.sub.x Ga.sub.1-x As (0.ltoreq..times..ltoreq.1) DH structure 5 is shown along with a bandgap energy diagram corresponding to the composition of each layer. When cladding layers 2 and 3 are forward biased and current is applied greater than the lasing threshold, electrons are injected from the wide bandgap n-ternary cladding layer 2 into the lower bandgap p-binary active region 1 where they recombine radiatively with holes to produce stimulated emission at a wavelength approximately equal to the bandgap energy of the material in active region 1. The bandgap difference at heterojunctions 6 and 7 creates a potential barrier which confines electrons to active region 1. Stimulated light is confined by virtue of the index of refraction discontinuity between cladding layers 2 and 3 and active region 1. The index of the binary active region material is greater than the index of the ternary material of cladding layers 2 and 3, thus, active region 1 guides the stimulated light.
Heterojunctions 6 and 7 should have as few defects as possible since such defects act as nonradiative recombination centers and tend to reduce efficiency and increase lasing thresholds. One way of improving the quality of the heterojunctions has been by lattice-matching the materials. One reason why the Al.sub.x Ga.sub.1-x As-GaAs system has been so intensively studied is that the lattice constants for Al.sub.x Ga.sub.1-x As and GaAs are approximately equal for all values of x. Thus, high quality heterojunctions can be formed.
Despite the quality of the heterojunctions, however, there are other sources of nonradiative recombination such as bulk nonradiative current, mirror current, and interface recombination current which may affect the threshold current. Referring to FIG. 1, active region 1 of the prior art DH structure has two interfaces (6 and 7) at which nonradiative recombination occurs, i.e., one p-n heterojunction 6 known as an anisotype heterojunction and one p-p (or n-n) heterojunction 7 known as an isotype heterojunction. The interface recombination current occurring at these heterojunctions represents carriers lost to the light producing process. Furthermore, it may also have an effect on laser degradation. We have found that the interface recombination current occurring at the anisotype heterojunction is several times greater than at the isotype.